The present invention relates to medical imaging devices such as nuclear or gamma cameras, as used in nuclear medicine, such imaging devices utilizing separate views taken at different positions to compute a tomographically reconstructed cross-sectional slice image. Specifically, the present invention concerns an apparatus for reducing the effect of motion, between successive views, on the tomographic slice image. The present invention may be also employed in imaging systems other than those used in nuclear medicine, for example, in x-ray computed tomography. For clarity, the following disclosure is directed towards an emission tomographic system.
In one type of emission tomographic imaging system, the single photon emission computed tomography system (SPECT), a low level gamma ray emitter is injected into the body of the patient. The gamma ray emitter is of a type which is absorbed preferentially by an organ whose image is to be produced.
Gamma rays from the gamma ray emitter absorbed by the patient are received by a large area, planar, gamma ray detector. The gamma ray detector incorporates a collimator to receive such radiation predominantly along a direction nominal to its plane. The intensity of the radiation over the plane of the detector is as an array of rows and columns of elements called pixels. Typically, the number of rows will be equal to the number of columns and there are commonly 64 or 128 pixels in each row and column.
A simple projection image may be formed from the array of pixels by assigning each pixel with an image brightness value corresponding to the intensity of the received radiation. Often, however, a tomographic image, or image of a slice through the patient, is preferred.
In tomographic imaging, multiple arrays of pixels are collected as the plane of the gamma ray detector is angled in equal increments in an orbit about an orbit axis through the patient. The two dimensional array of pixels obtained at each angle in the orbit is commonly termed a view. These multiple views are reconstructed according to tomographic reconstruction techniques to produce a set of slice images of adjacent planes perpendicular to the orbit axis. In turn, the reconstructed slice images together can be used to construct a variety of views along various planes within the volume imaged by the gamma ray detector.
Accurate tomographic reconstruction requires that the multiple views be combined so that corresponding pixels between views measure the intensity of radiation from the same volume elements or "voxels" of the patient's body, and that such corresponding pixels differ only in the angle at which the radiation is recorded. Generally, if the body being imaged changes position between successive views, e.g. by sliding along the orbit axis, the reconstructed images may be blurred or contain image artifacts or distortions.
Nevertheless, keeping the patient from moving during the collection of multiple views is not simple. In order to minimize the radiation dosage to which the patient is exposed, the injected gamma ray emitters are of relatively low radioactivity. As a consequence, each view can require up to 40 seconds to obtain. If a total of 64 views on a 360.degree. arc are desired, then the entire imaging process can require more than 40 minutes to complete.
Further, even a small amount of motion can effect the correspondence between pixels of successive views: a single pixel may be as small as 1/2 centimeter square. Thus, deleterious patient motion and resultant image degradation are common.
The existence of motion between views can usually be detected by examining projection images of the views in rapid succession ("cine mode"). In this case, the movement appears as a sudden jump in an apparently rotating projection image of the patient. Although such an approach may determine whether the collected data is usable, it does not provide a way to eliminate the motion if significant motion exists. Some correction may be performed by visually shifting the images to improve their alignment, but this approach is time consuming and unreliable.
Alternatively, an automated method of detecting and correcting for motion between views is described in U.S. Pat. No. 4,858,128 "View-to-View Image Correction for Object Motion" to Nowak, assigned to the same assignee as that of the present invention and hereby incorporated by reference. In this method, the two dimensional arrays produced in multiple views are collapsed to single one dimensional arrays. The collapsed arrays are then mathematically cross-correlated and the cross-correlation used to determine the relative motion between the views. The motion may then be compensated for by shifting some views with respect to the other views.
Although this automated method generally performs well, it has been determined by the present inventors that it requires the entire region of activity, from which gamma rays are being emitted, to remain within the field of view of the planar gamma ray detector in all views. This requirement is readily met when imaging compact organs, but is not met with images of larger structures such as the brain. In particular, in imaging the head, there can be significant gamma ray emission from the lower portion of the head and from the neck, such regions often extending out of the bottom of the field of view of the planar detector.
It is now recognized that the spatial truncation of the region of activity forms an abrupt discontinuity which is given undue weight in the cross-correlation process and which tends to disproportionately and erroneously influence the determination of motion between views. Generally, the cross-correlation process is misled by the edge of the planar detector, which doesn't move, to the exclusion of pixel data indicating actual patient movement.